Vacuum gauge, economy meter - Stewart Warner Motor Minder. I think I’m going to put this on the ‘68 Saab since it has a manifold vacuum port that’s not doing anything.
Vacuum gauges are wonderful for diagnosing engine issues, they can detect vacuum leaks, low compression, plugged exhaust, sticking valves and all other sorts of defects if you know how to read them. In a healthy engine though they’re a good indicator of engine load and throttle position, which is why quite a lot of these were sold in the 60s and 70s as economy meters - encouraging drivers to back off the gas just a little more to keep the manifold vacuum high (ie. to keep the throttle closed as far as possible for a given speed). I’m not sure it’s particularly effective in around town driving, but on the highway you might be surprised by how often you’re cruising with more throttle than really needed to maintain speed.
I’ve always wanted one, and it seems like an appropriate accessory for a car that was definitely being driven around during the early 70's oil crisis.